1. Field of the Invention
The present invention relates to a projection lens; and, in particular, to a projection lens used for a color liquid-crystal type projector which combines a plurality of images having color information items different from each other and then projects thus obtained composite image onto a screen surface under magnification, and a projector apparatus.
2. Description of the Prior Art
There have conventionally been proposed various kinds of color liquid-crystal type projectors adapted to optically superpose images displayed on a plurality of liquid-crystal displays (liquid-crystal light valves) and project thus obtained composite image onto a screen surface through a projection lens.
Here, an example of color liquid-crystal type projector will be explained with reference to FIG. 4.
As shown in FIG. 4, this color liquid-crystal type video projector comprises a light source 100; a pair of lens arrays 3A, 3B; first and second dichroic mirrors 5, 6 for separating the light from the light source into three primary colors; a condenser lens 4; first, second, third, and fourth total reflection mirrors 12, 14, 17, 18; three liquid-crystal display panels 7 (for red light), 8 (for green light), and 9 (for blue light); and a dichroic prism 10 for combining the three primary color light components; whereas a projection lens 11 is disposed on the exit side of the dichroic prism 10.
The light source 100 is a high-luminance white light source such as halogen lamp, metal halide lamp, or the like.
The first dichroic mirror 5 is a mirror for reflecting red light, whereas the second dichroic mirror 6 is a mirror for reflecting green light. Each of them comprises a glass substrate and a dichroic film provided thereon which is constituted by a dielectric multilayer film having such a spectral characteristic that it reflects a predetermined primary color light component as a mirror.
Each of the three liquid-crystal display panels 7, 8, 9 comprises a liquid-crystal display device of a twisted nematic type (TN type, STN type, of TFT type) or the like, and displays an image in response to its corresponding liquid-crystal signal from a liquid-crystal driver which is not shown, thereby modulating the respective incident primary color light component in terms of luminance.
The dichroic prism 10 is constituted by four rectangular prisms which are cemented together. Its two cemented planes orthogonal to each other are provided with respective dichroic films having spectral characteristics as a red-light-reflecting mirror and a blue-light-reflecting mirror, whereby three primary color light components of red, green, and blue can be combined into a single luminous flux.
A field lens 20 and a relay lens 21 are disposed in a system for blue light, whereas field lenses 15, 16, 19 are disposed in front of the liquid-crystal display panels 7, 8, 9, respectively.
The projection lens 11 employed in thus configured color liquid-crystal type video projector is configured so as to be able to project the combined three primary color light components onto a screen disposed at a predetermined distance, such that a full-color image is projected on this screen under magnification.
The following properties are required for this projection lens 11.
First, since light beams are combined by the cross dichroic prism 10, the projection lens 11 is required to have a low chromatic aberration, a long back focus, and a telecentricity. Also, for yielding a large projection image at a short projection length, the projection lens 11 is required to have a wider angle. Further, for projecting the quadrangular liquid-crystal display panels 7, 8, 9 without distortion, the projection lens 11 is required to reduce its distortion.
Thus, for projection lenses using liquid-crystal display panels as such, an optical system having a telecentricity in relation to the liquid-crystal display panels or illumination systems and a long back focus necessary for inserting the color-combining optical system and the like have been in demand (see Japanese Unexamined Patent Publication No. 10-260346, for example). On the other hand, as color liquid-crystal type projectors have been attaining higher resolution and higher luminance recently, there have been demands for developing bright projection lenses having high performances.
As a projection lens attains a higher resolution and a higher brightness, however, the lens inevitably becomes larger, whereby the mechanism for moving the lens group upon a focus adjustment also becomes larger.
In the field of imaging lenses for single-lens reflex cameras and video cameras, an inner focus type in which only a part of lenses in a lens group is made movable so as to realize a simple focus adjustment has been known. If this inner focus type is applied to the projection lens as it is, however, the exit angle will be so acute that telecentricity may not be obtained.
Thus, when such an inner focus type lens is employed in a color liquid-crystal type projector, there is a problem that color unevenness may occur on the screen when combining colors in the cross dichroic prism 10. Therefore, it is difficult for a conventional inner focus type lens to be used as the projection lens in a color liquid-crystal type video projector as it is.
Also, while a long back focus has been demanded in order to insert a color-combining optical system and the like, the system as a whole becomes larger if the back focus is too long, whereby the lens diameter on the reduction side may increase.